Bmi1 facilitates primitive endoderm formation by stabilizing Gata6 during early mouse development

Abstract

The transcription factors Nanog and Gata6 are critical to specify the epiblast versus primitive endoderm (PrE) lineages. However, little is known about the mechanisms that regulate the protein stability and activity of these factors in the developing embryo. Here we uncover an early developmental function for the Polycomb group member Bmi1 in supporting PrE lineage formation through Gata6 protein stabilization. We show that Bmi1 is enriched in the extraembryonic (endoderm [XEN] and trophectodermal stem [TS]) compartment and repressed by Nanog in pluripotent embryonic stem (ES) cells. In vivo, Bmi1 overlaps with the nascent Gata6 and Nanog protein from the eight-cell stage onward before it preferentially cosegregates with Gata6 in PrE progenitors. Mechanistically, we demonstrate that Bmi1 interacts with Gata6 in a Ring finger-dependent manner to confer protection against Gata6 ubiquitination and proteasomal degradation. A direct role for Bmi1 in cell fate allocation is established by loss-of-function experiments in chimeric embryoid bodies. We thus propose a novel regulatory pathway by which Bmi1 action on Gata6 stability could alter the balance between Gata6 and Nanog protein levels to introduce a bias toward a PrE identity in a cell-autonomous manner.

Gata6, Nanog and Erk signaling control cell fate in the inner cell mass through a tristable regulatory network

“…This phenomenon has been partially observed by immunofluorescence in independent studies (Gerbe et al, 2008;Plusa et al, 2008;Lavial et al, 2012). However, these global analyses did not quantify Nanog and Gata6 cell-to-cell variation at different stages.…”

“…Quantification of Gata6 protein indeed demonstrates a low decrease, with 19.3% at E3.25 and 32.0% at E3.75. Various factors could explain why the activity of Gata6 is not reduced by 50% in Gata6 +/− embryos: an increase in allelic transcription; a regulation of Gata6 mRNA targeted degradation (Elatmani et al, 2011); post-translational modifications, such as phosphorylation (Adachi et al, 2008); or modulation of protein degradation through the binding of Bmi1 (Lavial et al, 2012). Altogether, the analysis of Gata6 heterozygous embryos coupled to modeling reveals that a slight diminution of one factor can greatly perturb the system, demonstrating that precise levels of Gata6 and Nanog are crucial for the balance between PrE and Epi fates, and the timing of their specification.…”

“…When multiple stable steady states co-exist, the outcome of the system depends on the initial conditions. In the embryo, Nanog and Gata6 proteins start to be detected at the 8-cell stage and, at the 32-cell stage, they are co-expressed in almost all blastomeres (Plusa et al, 2008;Lavial et al, 2012). In the model, which involves arbitrary time units, the beginning of the simulations corresponds to the stage at which Nanog and Gata6 are null (Fig.…”

Polycomb subunit BMI1 determines uterine progesterone responsiveness essential for normal embryo implantation

“…However, we surprisingly found that BMI1 deficiency exerted apparently no effects on the overall levels of H2AK119 monoubiquitination in both mouse uteri and human endometrial cells. Increasing evidence suggests that BMI1 can function in a polycomb complexindependent manner in various processes (30,31). Therefore, it is conceivable that BMI1 may function in a noncanonical manner independent of PRC1 during early pregnancy.…”

Arf tumor suppressor and miR-205 regulate cell adhesion and formation of extraembryonic endoderm from pluripotent stem cells

“…1C). Bmi1 is essential for silencing of the entire Ink4-Arf locus in adult hematopoietic and neural stem cells and is required for formation of the early ExEn lineage (12), where, in contrast, it does not interfere with p19 Arf expression (Fig. 1C).…”